The present invention relates to a control device of an electric power steering which is installed in a vehicle, such as an automotive vehicle, and specifically relates to an electric power steering control device capable of suppressing both resonance of a vehicle-body member, such as a sub frame, and a vibration due to a rotation of a tire, such as shimmy.
The electric power steering installed in the vehicle, such as the automotive vehicle, comprises a motor to apply an assist torque to a steering device, a torque sensor to detect a steering torque applied by a driver, a vehicle speed sensor to detect a vehicle speed, and a control unit (ECU). The ECU sets a torque to be outputted by the motor (a motor torque) based on the steering torque detected by the torque sensor and the vehicle speed detected by the vehicle speed, and controls a current to be applied to the motor so as to provide the set motor torque (which is called an assist control).
Conventionally, the ECU simultaneously performs a vibration suppression control to suppress a vibration due to disturbance, such as sympathetic vibrations, in addition to the above-described assist control. The reason for this is that if the vibration suppression control is not performed, the above-described vibration is not only transmitted to driver's hands but a torque due to the above-described vibration is increased by the assist control.
U.S. Pat. No. 8,626,394 B2 discloses, as an example of the vibration suppression control, a device which comprises an assist map to output an assist torque current (a current to be applied to a motor) based on a steering torque applied by a driver, a vibration extracting filter to output a vibration component signal by decreasing a low-frequency side gain through performing filter processing on a rotational speed of the motor, a current variable gain map to calculate a current variable gain based on the current flowing through the motor, and a rotational-speed variable gain map to calculate a rotational-speed variable gain based on the rotational speed of the motor, thereby calculating a vibration suppression current based on the vibration component signal, the current variable gain, and the rotational speed variable gain, and then correcting the assist torque current by using the calculated vibration suppression current.
Meanwhile, there occurs the shimmy (tire shimmy) as one of vibrations experienced often during driving of the vehicle. The shimmy is the vibration due to an improper wheel balance and caused by a tire exchange, correction of the wheel balance, or the like. For example, when a vehicle travels at about 100-120 km/h (with a tire rotation frequency of about 10 Hz) on a highway, a steering wheel may vibrate slightly and quickly, which is the shimmy.
A vibration occurs inside a suspension device supported at a front sub frame due to the rotation of the tire, and this occurring vibration (the vibration due to the tire rotation) is transmitted to a steering wheel through a steering device including a tie rod, a pinion-rack mechanism, a steering shaft, and so on. The shimmy occurs accordingly. The suspension device comprises many kinds of members, each member of which has a natural frequency which is unique and different. Some of the above-described many kinds of members resonate with the rotation frequency of the tire and plural resonations of these members are combined and transmitted together, thereby generating the shimmy. Therefore, there are plural members resonating when the shimmy occurs, and thus it is not easy to find out the member which causes the shimmy.
Herein, since the shimmy occurs when the rotation frequency of the tire increases to a specified frequency and the vibrations of the plural members of the suspension device which resonate at this specified frequency are combined and transmitted to the driver's hands through the steering device, it is clear that the shimmy occurs when the vehicle speed increases up to a specified speed. Further, the rotation frequency of the tire where the shimmy occurs (a shimmy-occurrence frequency of 10 Hz in the above-described example) depends on vehicles which have individuality, different repair history, and the like, or aged deterioration of the suspension device, for example, of a vehicle itself. That is, the vehicle speed which causes the shimmy is unpredictable, so that it is necessary to have investigated this vehicle speed in advance.
The vibrations which occur due to the rotation of the tire described above include the one which is caused by deformation of a disc plate of a disc brake except the shimmy.
Further, in this case, if the natural frequency of the front sub frame of the vehicle-body member which supports the suspension device is within a range of the shimmy occurrence frequency (7-14 Hz, for example), when the tire rotation frequency increases up to the shimmy-occurrence frequency, the sub frame resonates with the tire rotation frequency. Since the sub frame is a large-sized and heavy member, the resonance of the sub frame is disturbance to be suppressed by the vibration suppression control. And, since the natural frequency of the sub frame does not change very greatly because of aged deterioration or the like, the tire rotation frequency with which the sub frame resonates (a sub-frame resonation frequency) is not changeable but steady. This kind of resonation of the vehicle-body member includes resonance of a side frame which is connected to the sub frame, resonance of a cross member which is connected to the side frame, and the like.